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badjester

particle opacity to isoOffset

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Hi All

I am birthing some particles that have a color node at the end. In color and alpha im using the ramp with the $LIFE attr to control the particles. Over time the alpha fades out and the color changes. I copy spheres (tried all types but using polys) with a copy sop. In the copy sop i am using "Use Template Point Attr" to send down the color and alpha from the color node in the popNet. The color and alpha works fine, some weird artifacts are happening on the geo like missing polys on the spheres (not the main problem). then when i use a isoOffset to convert spheres to volume, I use a attr Transfer to bring back the color and alpha but only the color works. So when my smoke births in and dies out the volumes are popping. I want to keep that alpha control that I had in popNet for smooth birthing and dieing. Any help would be great.

----Badjester----

testVolume.hip

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I think there's no "Alpha" in the volume context. What you need to do is to use alpha as a density field. I tried to build you a hip file but I wasn't able to make a density field out of alpha value thanks to the horrifyingly unintuitive world of volume processing. Maybe someone with more than 6 years of Houdini experience can do this for us.

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Just brainstorming here since I don't know volume manipulation very well. But another thing you could do is transfer the life attr to the voxel grid and populate the density based on that (some 0-1clamping formula that converts the contrary of the life to the density), or something along those lines.

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hey thanks for the replies. I am going to try converting Alpha to density, i think i have an example file that has a similar conversion from school.

I am not sure were to start with converting the $LIFE attr to a density in voxels. Ill do some more search and post anything that starts to work.

Thanks

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hey thanks for the replies. I am going to try converting Alpha to density, i think i have an example file that has a similar conversion from school.

I am not sure were to start with converting the $LIFE attr to a density in voxels. Ill do some more search and post anything that starts to work.

Thanks

Be sure to post a hip file here so we can all learn how this really is done. 40 minutes wasn't enough for me to figure it out.

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Oh and just because I had to figure this out on my own, VolumeFromAttrib will use pscale if it's present on the points so it's an easy way to control the sampling radius, plus it gives you falloff. Turning off Extrapolate exposes the Threshold and Bandwidth parameters, these are wonky/mathy names for Falloff and Scale. Raising Threshold will increase the amount of falloff from the center of the point, raising Bandwidth will add to the sampling radius. If you do not have pscale on your points you can either use Extrapolate which will generally give you chunky voxels, or turn it off and run the Bandwidth up until you hit the desired radius. If you need varying scale you have to use pscale as Bandwidth acts on all points being sampled.

Just out of curiosity, what is your preferred software for working with volumes? I have only really used Houdini and Naiad for volume work but I'd be interested to hear what else is out there :)

Edited by tjeeds

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I took the liberty to modify Jesse's file with a node that adjusts volume box size automatically. Amazingly this worked in SOP context so I thought I share it.

alpha_to_density_auto.hip

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I took the liberty to modify Jesse's file with a node that adjusts volume box size automatically. Amazingly this worked in SOP context so I thought I share it.

Ah once again I simply missed an operator with a promising name. Now how do you append the red color on top of that ?

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hey thanks for posting. I am getting a error when i load either of the scenes. Its failing to load certain parts so its "skipping unrecognized parameter"... and "Bad node type found". Basicaly im getting just particles. Im in houdini 10 if that helps. I really wanna see with what you did.

Thanks

Pat

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hey thanks for posting. I am getting a error when i load either of the scenes. Its failing to load certain parts so its "skipping unrecognized parameter"... and "Bad node type found". Basicaly im getting just particles. Im in houdini 10 if that helps. I really wanna see with what you did.

The 10 is your problem. The scene uses "Volume From Attribute" operator which I believe didn't exist before H11.

Edited by Hazoc

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I know the thread is old, but I am trying to reconstruct this same method in 10 without the use of volume from attribute (passing a custom velocity into my volume). I've done the same thing in 11, and its fairly easy and works great. Where I'm working we are still running H10. Any ideas? Thanks!

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You should be able to use point clouds in a volume vop instead of volume from attrib. H10 don't wanna boot on my system tho so can't see what is actually available to use.

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